The invention deals with the field of fluid dynamics. It relates to a gas turbine seal, comprising a metallic component with a durable or erosion-resistant ceramic coating and an abradable ceramic layer which is arranged thereon in locally delimited fashion. Coated components of this type are used, for example, for sealing and thermal insulation between rotating and stationary components in gas turbine installations.
It is known that, to increase the efficiency of thermal turbomachines, as far as possible there should be only small clearances between the housing and the rotating blade or vane tips in the compressor and in the turbine, in order to minimize or prevent leaks of air or combustion products between the two components, and thereby to keep losses at a minimum.
One possible way of reducing such leaks is for the tolerances between the stationary and rotating components to be kept as tight as possible during manufacture and assembly of the components. However, this has the drawback that, firstly, the tighter the tolerances between the corresponding components, the higher the costs become and, secondly, on account of the different thermal expansion and contraction of the components interactions take place between these components before, during and after operation, causing wear and other damage to the components.
For this reason, various sealing systems have been developed, for example run-in layers, also known as abradables, which are applied to the surface of the stationary components and into which the rotating components grind during operation.
Known run-in layers are, for example, the classic honeycombs. These comprise thin metal sheets which are soldered on in honeycomb form, are abraded by graning and therefore allow minimal play between the rotating component, for example a rotating blade or vane tip of a gas turbine, and the stationary component, e.g. a stationary housing element of a gas turbine. Honeycombs have web heights of approx. 3 mm to 12 mm and more, so that the rotating counterpart can grind into the run-in layer down to a depth of up to several millimeters.
Since the 1980s, it has also been attempted to produce ceramic flame-sprayed or plasma-sprayed run-in layers, since the turbine inlet temperatures have been increased in order to raise performance and improve efficiency, and for this reason the metallic substrates which are exposed to the high temperatures had to be provided with ceramic protective layers.
It is known from the publication by A. Sickinger and J. Sohngen: Development of thermal spray layers as gas path for aircraft turbine engines. International Thermal Spray Conference, Essen, 02.05.1983 for ceramic material to be sprayed into the honeycombs. In the process, a bond layer with a surface which is as rough as possible, in this case comprising Nixe2x80x94Crxe2x80x94Al, is sprayed onto the metallic surface of the base body, for example by means of plasma spraying or flame spraying. The roughness of the surface is used to provide positively locking anchorage for the thermal barrier coating (TBC), comprising a non-metallic material, in this case ZrO2xe2x80x94CaO, which is subsequently likewise plasma-sprayed or flame-sprayed onto this surface. However, on account of the very different coefficients of thermal expansion between metals and non-metallic materials, such as ceramics, these joins are usually only successful up to a layer thickness of  less than 500 xcexcm. Then, an intermediate layer of ZrO2xe2x80x94CaO+Nixe2x80x94C is applied, onto which, finally, the run-in layer comprising Nixe2x80x94C is thermally sprayed. Ceramic layers of this type in the honeycombs have in practice not been used in practice for high loads.
EP 0 965 730 A2 has disclosed a gas turbine air seal, in which, first of all, a thin bond layer comprising aluminum oxide is applied to a base material. In turn, a layer of durable or erosion-resistant ceramic material (TBC) is arranged on this aluminum oxide layer, and finally an abradable ceramic material (run-in layer) is applied to the TBC in locally delimited fashion. A drawback of this arrangement is that it is only possible to achieve relatively small layer thicknesses, which are often insufficient in gas turbine installations, primarily in the final compressor stages or in the turbine stages with a view to achieving a required thermal barrier or required grinding away.
Although it is nowadays also possible, with particularly high levels of outlay, to spray ceramic layers which are approx. 1 mm to 2 mm thick, these layers react very sensitively to the action of external forces. However, particularly in the case of run-in layers, it is necessary to reckon with a relatively substantial stripping action. The introduction of radial/tangential forces, sickle-shaped contact or even locally high overheating/frictional heat cause the ceramic to flake away quickly if the counterpart is not abrasive enough and cuts in quickly.
Therefore, to ensure sufficient bonding without the abovementioned drawbacks of thick ceramic layers on a base body, very coarse holding structures have to be produced on the surface of the base body.
The applicant is aware of various processes, cf. for example DE 100 57 187.5, in which spherical or mushroom-shaped coarse holding structures (anchor points, also known as rivets) for ceramic materials which are to be applied are produced on a metallic surface by a welding or casting process. These ceramic materials are primarily thermal barrier coatings which are used, for example, in gas turbine combustion chambers and are constantly exposed to high thermal loads and sometimes local impact loads.
The invention attempts to avoid the abovementioned drawbacks of the known prior art. It is based on the object of developing a gas turbine seal comprising a metallic component with a durable or erosion-resistant ceramic coating, for example a thermal barrier coating, and an abradable ceramic layer which is arranged thereon in locally delimited fashion, in which, despite high layer thicknesses of the ceramic of up to approx. 20 mm, a good bond strength is to be achieved, ensuring that the ceramic does not flake away even in the event of the introduction of radial/tangential forces, sickle-shaped-contact or locally high overheating/frictional heat.
According to the invention, in a gas turbine seal in accordance with the preamble of patent claim 1, this is achieved by the fact that the bond layer comprises separate, adjacent spherical rivets or mushroom-shaped rivets which have a web and a head.
According to the invention, the gas turbine seal system comprises a gas turbine seal according to the invention and a rotating component, preferably a rotor blade, which grinds into the abradable ceramic layer.
An advantage in this respect is that with the gas turbine seal according to the invention high ceramic layer thicknesses of up to approx. 20 mm are achieved, these layers having a good bond strength, and the ceramic not being flaked away even in the event of the introduction of radial/tangential forces, sickle-shaped contact or locally high overheating/frictional heat, and a good sealing action being achieved.
Moreover, the use of rivets as a bond layer has the effect of forming a continuous ceramic network with individual metal islands, which has a positive effect on the properties of the layer. For example, in particular the lower heat conduction, the smaller metal surface area exposed to oxidation and the improved anchoring of the ceramic layer achieved with the holding structures according to the invention compared to the network-or lattice-like holding structures which are known from the prior art should be mentioned.
Advantageous configurations of the gas turbine seal are disclosed in subclaims 2 to 12.
It is advantageous that both the layer thickness of the abradable ceramic layer (run-in layer), at approx. 1 to 8 mm, and that of the durable or erosion-resistant ceramic layer (generally plasma-sprayed thermal barrier coating TBC), at 1 to 20 mm, are relatively high compared to the known prior art. This is possible only on account of the excellent possibilities for anchoring the ceramic at the undercuts of the rivets. The height of the rivets can be accurately matched to the particular requirements. It is also possible for the TBC layer to be matched to the particular load conditions, for example it may be of graduated (differing material, i.e. differing ceramics and/or sprayed in a differing density) or ungraduated structure. Finally, the metallic component may also be additionally coated with an oxidation protective layer, to which the TBC layer is then applied.